Efficient coding of speech signal means not only getting a high quality digital encoding of the signal but in addition optimizing cost and coder complexity.
In some already known coders, the original speech signal is processed to derive therefrom a speech representative residual signal, compute a residual prediction signal using Long-Term Prediction (LTP) means adjusted with detected pitch related data used to tune a delay device, then combine both current and predicted residuals to generate a residual error signal, and finally code the latter at a low bit rate.
A significant improvement to the above cited type of coding scheme efficiency was provided, in copending European Application (EP 87430006.4), by detecting the pitch or an harmonic of said pitch (hereafter simply referred to as pitch, or pitch representative data, or pitch related data) using a dual-steps process including first a coarse pitch determination through zero-crossings and peak pickings, followed by a refining step based on cross-correlation operations performed about the detected pitched peaks.
While being particularly useful, the above cited pitch tracking process involves a rather high computing load as compared to the overall coder computing load.
For instance, using presently available signal processors, one had to devote 0.7 MIPS over 4 MIPS involved for an RPE/LTP coder just to pitch tracking operations.